In an electrostatographic process, a system is used whereby a uniform electrostatic charge is placed upon a reusable photoconductive surface. The charged photoconductive surface is then exposed to a light image of an original to selectively dissipate the charge to form a latent electrostatic image of the original on the photoreceptor. The latent image is developed by depositing finely divided marking and charged particles (toner) upon the photoreceptor surface. The charged toner is electrostatically attached to the latent electrostatic image areas to create a visible replica of the original. The toned developed image is then transferred from the photoconductor surface to a final image support material, such as paper, and the toner image is fixed thereto by heat and pressure to form a permanent copy corresponding to the original.
In Xerographic systems of this type, a photoreceptor surface is generally arranged to move in an endless path through the various processing stations of the Xerographic process. The photoconductive or photoreceptor surface is generally reusable whereby the toner image is transferred to the final support material, and the surface of the photoreceptor is prepared to be used once again for another reproduction of an original. In this endless path, several stations of corona charging are traversed. These charging stations may involve one or a cluster of dicorotron or other corotron units.
Several methods are known for applying an electrostatic charge to the photosensitive member such as the use of electron-emitting pins, an electron-emitting grid, single corona-charging structures and single or multiple dicorotron wire assemblies. In recent development of high speed Xerographic reproduction machines where copiers can produce at a rate of or in excess of three thousand copies per hour, the need for several reliable dicorotron wire assemblies and good final copies are required.
Usually, in electrostatographic or electrostatic copy processes, as those above noted, a number of corotrons or dicorotrons are used at various stations around the photoreceptor. For example, the dicorotrons are used at the station that places a uniform charge on the photoreceptor, at a transfer station, at a cleaning station, etc. In today's high speed copiers, it is important that all corotrons (or dicorotrons) are somewhat controlled regarding effluents given off in the charging process. Generally, the structure of a dicorotron uses a thin, glass-coated wire mounted in an elongated U-shaped housing between two insulating anchors called “insulators”. These support the wire in the U-shaped housing in a spring-tensioned manner in a singular plane. This dicorotron unit or assembly, usually an aluminum, comprises housing, as above noted, is in one embodiment an elongated U-shaped shield. The wire or corona-generating electrode is typically a highly conductive, elongated wire situated in close proximity to the photoconductive surface to be charged.
As earlier noted, the charging of the photoreceptor is necessary for the proper operation of the Xerographic machine. A by-product of corona charging devices are several gasses (most notably NOx and ozone) which are referred to in this discussion as “effluents”. The effluents can interact with the surrounding atmosphere, which may include organic compounds like morpholine, and with the photoreceptor itself to produce substantial negative effects on the photoreceptor and the resulting copy. These are sometimes called lateral charge migration (LCM) and/or parking deletion. This can cause the output of a printed copy to appear blurry or have areas where the image is entirely missing (deleted). Currently, fans and special coatings are utilized to remove or neutralize the gasses to various degrees of success.
Nitric oxide deletions and other effluents have been a pervasive and persistent problem in these electrostatic copying systems. The embodiments of this invention are simple and effective ways to minimize these problems.
Charging devices historically come in three forms; corotrons, scorotrons and dicorotrons; all will be referred to in this disclosure as “corotrons” or a source of “corona” discharge. The charging devices use high voltages to create a corona. This corona can be thought of as a collection of ions (charged atoms or molecules) in a local area. In most cases, the corona is influenced to move towards the desired target by the opposite charge on a screen or grid-type device.
The different names of the charge device or corotrons denote different configurations. Corotrons are simply bare wires. A high DC potential is placed on the corotron to create the corona. To charge photoreceptors to a positive voltage, a large positive DC voltage is placed on the corotron wire. To charge negatively, a negative potential is placed on the wire. Dicorotrons are a wire device also. In this case, the wire is coated with a thick film of dielectric glass. Dicorotrons have an alternating voltage placed on them to create both positive and negative ions. A screen or shield with a DC bias directs the dicorotron's charge toward the photoreceptor. The grid or shield voltage determines the polarity and amplitude of the charge placed on the photoreceptor.
An important consideration is that there are many ways to charge photoreceptors. Some ways have a propensity for problems to occur while others have less of an issue. In relation to nitric oxide deletions, the AC devices (dicorotrons) and the negative DC devices have a higher probability of deletion problems.
As stated, the charge device is the originator of the nitric oxide parking deletion (or, for sake of clarity, deletion). The deletion process begins with the production of corona in normal atmosphere. Corona is a “cloud” of charged ions. Different types of corona contain different ions, H+ and N4+ are the major positive ions for both AC and DC devices. The negative ions NO3− and O3− (ozone) are the major ions in negative DC discharge and AC with airflow. AC devices (dicorotrons) also contain the following negative ions: O−, OH−, O2−, NO2−, CO3−.
The ozone (O3) and NOx (NO and NO2) occur in relatively large amounts. These compounds are also very reactive, chemically. NOx is known as Oxides of Nitrogen. While both gasses and morpholine can contribute to the deletion problem, NOx has been cited as the main culprit, hence the reference to Nitric Oxide Deletion.
Recent experiments show that the NOx output from a dicorotron operated at nominal voltage is entirely NO2. Charge device NO2 output is attributed to the presence of ozone in the charge device area. Ozone oxidizes NO to NO2.
The oxidation of NO to NO2 produces one photon of light at about 1200 nm. This occurs in about 20% of the oxidized NO2. As the molecule decays to a stable state, a photon is emitted with the peak excitation of 1200 nm. This is the basis for a Chemilumenesence Nitric Oxide detector sometimes used in the present embodiments to measure effluents.
Photoreceptors have been shown to be very sensitive to nitric acid-type compounds (HNO3 and HNO2). The nitric acid attacks certain molecules in the transport layer of the photoreceptor rendering them too conductive. This conductivity allows any developed charge on the photoreceptor to leak to ground in the area of the attack or spread in what is sometimes (mistakenly) called lateral charge migration. Lateral charge migration is a separate issue involving the deposit of conductive salts on the photoreceptor through the interaction of corona and atmospheric contaminants, such as morpholine. In Nitric Oxide deletions, in the worst cases, areas near the acid attack appear blank on a copy because toner is not developed to the photoreceptor in those areas. In lesser extent cases, the problem manifests itself as a blurring of the image. Some volatile organic compounds, such as morpholine and organic nitrates are effluents also detrimental to the photoreceptor.
Nitric oxide deletions are often termed parking deletions. This nomenclature arises from the way in which nitric oxide deletions are most prevalent. When charging devices are run for a long period of time (during a long print run) a relatively large amount of NOx and O3 (as above indicated, collectively known as effluents) are built up. The effluents become adsorbed on the surface of nearby solids. When the machine is shut down, the photoreceptor stops rotation and becomes “parked” with a small area directly adjacent to the charge device. Over a short period of time, the adsorbed effluents are released from the charge device in a process known as outgassing. Since the photoreceptor is parked in very close proximity to the charge device, a small local area of the photoreceptor becomes damaged.
The embodiments of the present invention provide strategies employed to combat and minimize these deletions.
It is known to use Titanium to help chemically reduce effluents around a photoreceptor. (By virtue of its native oxide surface layer). It is also known to use Titanium Dioxide (TiO2) to remove nitric oxides from the environment via titanium dioxide coatings. See articles “Reactive Oxygen Species inhibited by Titanium Dioxide Coatings” (Suzuki et al.)) R. Suzuki, J. Muyco, J. McKittrick, J. McKitrick, J. Frangos. J. Biomed. Mater. Res. A, 2003, Aug. 1 vol. 66 No. 2: pg. 396-402), and “Titanium Dioxide: Environmental White Knight”, L. Frazer Environmental Health Perspectives Vol. 109, No. 4, April 2001. It is important to make the distinction between the metal Titanium and the ceramic titanium dioxide. Neither Suzuki nor Frazer suggests use of their process on electrostatic marking system nor suggest the problems of effluents in said systems. Suzuki et. al does not use a thermal plasma spray process; therefore, only provides a titanium coating of from 100-200 nanometers thick. Suzuki's thin coating of elemental titanium was produced by a process that would take an inordinate amount of time (days) to build a coating, as in the present embodiments, of about 20-100 microns and his process must be done in a vacuum. L. Frazer's article does suggest the use of a thermal plasma spray; his process is incapable of producing a coating of TiO2 that is relatively thick (at least 20 microns) and has a porosity of at least 1% of the coating. Also, a pore size of 0.1 micron would be difficult with Frazer's process. An important process difference between the present invention and the process of Frazer is that this invention uses UV energy supplied by the dicorotron as the photo source in the photocatalytic reaction, Frazer uses the UV energy from the sun.
Prior art attempts involved depositing elemental Titanium onto aluminum using physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods such as RF plasma deposition. The titanium metal would then form a native oxide layer by exposure to ambient environment. Unfortunately, these coatings were too thin and were quickly destroyed by the aggressive corona environment. Coatings made by this method are typically 100-200 nm thick. Titanium sheet metal was also tried. Using titanium sheet metal to control effluents was very costly and it was difficult to form titanium sheet metal into the complex shapes required. Methods for the deposition of titanium dioxide ceramic exist (such as sol-gel deposition), but are either considered too expensive or unreliable. Further, the sintering temperatures of titanium dioxide ceramic are too high to allow the material to be directly formed on aluminum. Plasma spray is the most effective alternative when one wishes to place a ceramic coating, such as titanium dioxide on a metal substrate, such as aluminum.